Field effect transistor and method of producing the same

ABSTRACT

A field effect transistor having a high field effect mobility is provided which can be obtained by a simple method. The field effect transistor includes an organic semiconductor layer composed of a crystallized film of a naphthoporphyrin compound represented by formula (2), which is obtained by the conversion by heating of the coating film of a porphyrin compound represented by formula (1), the organic semiconductor layer having crystal grains with a maximum metediar of 1 μm or more, wherein R 1  and R 2  each independently denote at least one selected from the group consisting of hydrogen, halogen, hydroxyl, and alkyl having 1 to 12 carbon atoms; R 3  denotes at least one selected from the group consisting of a hydrogen atom and an aryl group; and M denotes two hydrogen atoms, a metal atom or a metal oxide.

TECHNICAL FIELD

The present invention relates to a field effect transistor and a methodof producing the same, particularly to a field effect transistor whichcan be easily produced and has a high mobility and a method of producingthe same, by using, as an active layer, a crystallized film of anaphthoporphyrin compound which is obtained by heating a coating film ofa porphyrin compound soluble in organic solvents.

BACKGROUND ART

Non-linear optical properties, electrical conductivity andsemiconductivity of organic semiconductor compounds have receivedattention in organic electronics and optoelectronics fields, and thedevelopment of various devices has been advanced actively. Typicalexamples of the organic semiconductor compounds include phthalocyaninecompounds, porphyrin compounds and polyacenes. Properties such asnon-linear optical properties, electrical conductivity andsemiconductivity, which are required for using these compounds asorganic materials to form devices, largely depend on crystallinity andorientation other than only purity of the materials. However, it hasbeen difficult to highly purify these materials due to the reason thatmany compounds with extended π-conjugated systems are insoluble insolvents and susceptible to oxidation in air. In addition, a large-scaleapparatus has been required for forming films, for example, a vacuumdeposition apparatus for performing formation of a crystallized filmwith high orientation.

In recent years, organic field effect transistor (FET) devices usingorganic semiconductor compounds as the semiconductor layers havereceived attention. Organic semiconductor compounds, which show moreflexible film properties compared with the properties of inorganicmaterials such as silicon, have been regarded as suitable materials forfabricating flexible devices using plastics as substrates.

However, as described above, pentacene and the like that are typicalexamples of organic semiconductor compounds are hardly soluble insolvents due to high crystallinity, so the formation of their films onsubstrates has only been possible by vacuum deposition. On the otherhand, FETs are more easily fabricated by forming thin films by spincoating or the like from a solution of an organic semiconductor solublein organic solvents. As an example of the above, π-conjugated polymersare used for semiconductor layers (Refer to “Japanese Journal of AppliedPhysics”, Japan Society of Applied Physics, Vol. 30, p. 596-598, 1991).In the case of π-conjugated polymers, it is known that the state of theorientation of molecular chains largely affects the electricalconductive properties. Similarly, it is reported that the field effectmobility of π-conjugated polymer field effect transistors largelydepends on the state of the orientation of molecular chains insemiconductor layers (Refer to “Nature”, Nature Publishing Group, Vol.401, P. 685-687, 1999).

However, since the molecular chains of π-conjugated polymers areoriented in a period between the coating of a solution and the drying ofthe same, there has been a possibility in which the state of theorientation of molecular chains largely varies depending on the changeof environment or the difference of coating methods. There is alsoreported a FET using a film in which a soluble precursor thin film ofpentacene is formed by coating and transformed to pentacene byheat-treatment (“Advanced Materials”, WILLEY-VCH Verlag GmbH, Vol. 11,p. 480-483, 1999). In this case, the conversion to pentacene hasrequired a high-temperature treatment, and eliminated components with alarge mass must have been removed under a reduced pressure.

DISCLOSURE OF THE INVENTION

As described above, FET devices using organic semiconductors haverequired a complicated process such as vacuum film forming, or have hadproblems that they are susceptible to environment.

The present invention has been created to solve these problems, and itis an object of the present invention to provide a field effecttransistor with a high field-effect mobility, which includes an organicsemiconductor layer formed by a simple method, and to provide a methodof producing the same.

The present inventors have found that a crystallized film can be easilyobtained by heat-treating a coating film obtained when an organicsolvent solution of a porphyrin compound represented by general formula(1) is coated on the surface of a substrate, and that the field effecttransistor using this crystallized film as an organic semiconductorlayer exhibits the field effect mobility that is the same as or higherthan that of the organic semiconductor layer whose film formed byconventional vapor deposition. The present invention has been createdbased on these findings.

Namely, a method of producing a field effect transistor including anorganic semiconductor layer according to the present invention ischaracterized by comprising a step of heating a coating film comprisinga porphyrin compound represented by general formula (1):

wherein R₁ and R₂ each independently denote at least one selected fromthe group consisting of hydrogen, halogen, hydroxyl, and alkyl,oxyalkyl, thioalkyl and alkyl ester, each alkyl having 1 to 12 carbonatoms; R₃ denotes at least one selected from the group consisting of ahydrogen atom and an aryl group; and M denotes two hydrogen atoms, ametal atom or a metal oxide;to form, as the organic semiconductor layer, a crystallized film of anaphthoporphyrin compound represented by general formula (2):

wherein R₂, R₃ and M each denote the same as defined above.

Further, the above described coating film of a porphyrin compoundrepresented by general formula (1) is heated in a range from 200 to 350°C. to form the compound represented by general formula (2).

Further, the field effect transistor according to the present inventionis characterized by including an organic semiconductor layer composed ofa crystallized film of a naphthoporphyrin compound which is obtained byheating the coating film comprising a porphyrin compound represented bygeneral formula (1):

wherein R₁ and R₂ each independently denote at least one selected fromthe group consisting of hydrogen, halogen, hydroxyl, and alkyl,oxyalkyl, thioalkyl and alkyl ester, each alkyl those having 1 to 12carbon atoms; R₃ denotes at least one selected from the group consistingof a hydrogen atom and an aryl group; and M denotes two hydrogen atoms,a metal atom or a metal oxide,to convert the porphyrin compound to a naphthoporphyrin compoundrepresented by general formula (2):

wherein R₂, R₃ and M each denote the same as define above,to form the organic semiconductor layer composed of a crystallized filmof the naphthoporphyrin, the organic semiconductor layer having crystalgrains with a maximum diameter of 1 μm or more and having a strongabsorption at 650 nm or longer.

Each of R₁ and R₂ of the above porphyrin compound represented by generalformula (1) is preferably a hydrogen atom, and R₂ of thenaphthoporphyrin compound represented by general formula (2) ispreferably a hydrogen atom.

R₃ of the above porphyrin compound represented by general formula (1)and of the above naphthoporphyrin compound represented by generalformula (2) is preferably a hydrogen atom.

M of the above porphyrin compound represented by general formula (1) andof the above naphthoporphyrin compound represented by general formula(2) is preferably two hydrogens.

M of the above porphyrin compound represented by general formula (1) andof the above naphthoporphyrin compound represented by general formula(2) is preferably one copper atom.

The above organic semiconductor layer preferably has a field effectmobility of 1×10⁻³ cm²/V·s or more and an On/Off ratio of 100 or more.

The present invention can provide a field effect transistor with a highfield effect mobility by a simple method that is extremely simplecompared to conventional methods.

Other features and advantages of the present invention will be apparentfrom the following description taken in conjunction with theaccompanying drawings, in which like reference characters designate thesame or similar parts throughout the figures thereof.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are incorporated in and constitute apart of the specification, illustrate embodiments of the invention and,together with the description, serve to explain the principles of theinvention.

FIG. 1 is an enlarged schematic view showing a part of the field effecttransistor of the present invention; and

FIG. 2 is a graph showing electrical properties of the field effecttransistor in Example 1 of the present invention.

BEST MODE FOR CARRYING OUT THE INVENTION

Preferred embodiments of the present invention will now be described indetail in accordance with the accompanying drawings.

Hereinafter, the present invention will be described in detail.

The field effect transistor according to the present invention includesan organic semiconductor layer composed of a crystallized film of anaphthoporphyrin compound which is obtained by heating the coating filmcomprising a porphyrin compound represented by general formula (1):

wherein R₁ and R₂ each independently denote at least one selected fromthe group consisting of hydrogen, halogen, hydroxyl, and alkyl,oxyalkyl, thioalkyl and alkyl ester, each alkyl having 1 to 12 carbonatoms; R₃ denotes at least one selected from the group consisting of ahydrogen atom and an aryl group; and M denotes two hydrogen atoms, ametal atom or a metal oxide,to convert the porphyrin compound to form the organic semiconductorlayer composed of a crystallized film of a naphthoporphyrin compoundrepresented by general formula (2):

wherein R₂, R₃ and M each denote the same as defined above.

In the present invention, the substituent R₁ bonded to a dicyclooctaenering of the above porphyrin compound represented by general formula (1)is eliminated in the form of R₁—CH═CH—R₁ when the porphyrin compound isconverted to a naphthoporphyrin compound represented by general formula(2) by heating. Therefore, R₁ may be each independently at least oneselected from the group consisting of hydrogen, halogen, hydroxyl,alkyl, oxyalkyl, thioalkyl and alkyl ester, each alkyl having 1 to 12carbon atoms, or may be a combination of two or more thereof. If thenumber of carbon atoms exceeds 12, the molecular weight of an eliminatedcomponent increases and the eliminated component remains in anaphthoporphyrin compound film, resulting in inadequate semiconductorproperties. R₁ is most preferably a hydrogen atom.

The substituent R₂ of the porphyrin compound represented by generalformula (1) remains as a substituent in the naphthoporphyrin compoundobtained after heat treatment. Therefore, the substituent R₂ affects theorientation of naphthoporphyrin. R₂ may be each independently at leastone selected from the group consisting of hydrogen, halogen, hydroxyland alkyl, oxyalkyl, thioalkyl and alkyl ester, each alkyl having 1 to12 carbon atoms, or may be a combination of two or more thereof. If thenumber of carbon atoms of R₂ exceeds 12, the existence ratio of aporphyrin ring to the whole molecule is reduced to prevent theorientation of porphyrin rings, resulting in inadequate semiconductorproperties. Most preferably, R₂ is a hydrogen atom, wherein the stackingof porphyrin rings more easily occurs, which enhances the crystallinityof a film.

M of a porphyrin compound is not particularly limited, provided that Mis two hydrogens, metal or a metal oxide. The example of metal includescopper, gold, silver, zinc, nickel, chromium, magnesium, lithium orcobalt. The example of the metal oxide includes the oxide of iron,titanium or aluminum. Most preferably, M is two hydrogen atoms or onecopper atom.

A method of forming the organic semiconductor layer is preferablyincludes a method in which a porphyrin compound represented by generalformula (1) is dissolved in a solvent, coated on a substrate and thenheated to obtain a crystallized film of a naphthoporphyrin compoundrepresented by general formula (2).

The solvent to be used for dissolving the porphyrin compound is notparticularly limited, provided that it does not react with the porphyrincompound nor precipitate the same. Further, two or more solvents may bemixed for use. Halogenated organic solvents are preferably used inconsideration of smoothness of the surface of a coating film and theuniformity of a film thickness. The example of the halogenated organicsolvent includes chloroform, methylene chloride, dichloroethane,chlorobenzene or 1,2-dichloroethylene. The solution is arbitrarilyadjusted to any concentration depending on desired film thicknesses,preferably from 0.01 to 5% by weight.

A coating method includes a spin casting method, a dipping method, adropping method, a printing method such as an offset or a screenprinting or an ink jet method. Further, it is desirable to preliminarilyfilter the solution through a membrane filter in order to prevent theintrusion of dirt and the like into a semiconductor layer as much aspossible. Because insolubles and the intrusion of dirt from outsideprevent uniform orientation and cause the increase of the off currentand the reduction of the On/Off ratio. The coating film ofnaphthoporphyrin may also be preliminarily dried at 130° C. or lower.

The coated film of a porphyrin compound is subjected to a retroDiels-Alder reaction by heating to be converted to a naphthoporphyrincompound, which involves the elimination of R₁—CH═CH—R₁. At the sametime as the production of the naphthoporphyrin compound, the crystalgrowth is caused by the stacking of porphyrin rings, resulting in acrystallized film of the naphthoporphyrin compound. The eliminationreaction occurs at 200° C. or higher. However, the desired heatingtemperature for obtaining a higher field effect mobility is from 200 to350° C., preferably from 250 to 300° C. A crystallized film with anadequate crystal growth cannot be obtained at a temperature of lowerthan 200° C., and cracks will occur due to an abrupt shrinkage of thefilm when the temperature exceeds 350° C.

The heating is carried out on a hot plate, in a hot air-circulating ovenor in a vacuum oven. An instantaneous heating method on a hot plate ispreferred for obtaining uniform orientation. Further, in order to obtainhigher crystallinity, the coating film before-heat treatment ispreferably subjected to rubbing treatment in which it is lightly rubbedwith a cloth or the like. The cloth for use in the rubbing treatmentincludes, but not limited to, rayon, cotton or silk.

The desired film thickness of the organic semiconductor layer using theoriented film of the naphthoporphyrin compound obtained by theseoperations is in the range from 30 to 150 nm, preferably from 50 to 100nm. When the film thickness is less than 30 nm, the uniformity of thefilm thickness will be impaired. On the other hand, when it is exceeds150 nm, the mobility will be reduced due to the impairment of thesmoothness of a film surface.

The crystal growth can be verified by X-ray diffraction, film surfaceobservation using an optical microscope, a laser microscope or the likeand an ultraviolet-visible light absorption spectrum of the film. Thedesired maximum diameter of the crystal grains in the organicsemiconductor layer is in the range of 1 μm or more, preferably 2 μm ormore. If it is less than 1 μm, adequate field effect mobility cannot beobtained.

Further, it is preferable that a strong absorption is observed at 650 nmor longer in the ultraviolet-visible light spectrum of the organicsemiconductor layer. If no absorption or only a weak absorption isobserved at 650 nm or more, the stacking of porphyrin rings will be weakand will not be oriented adequately, resulting in the reduction of fieldeffect mobility.

The organic semiconductor layer obtained in the present invention has afield effect mobility of 1×10⁻³ cm²/V·s or more. If the field effectmobility is lower than the above value, the current value between asource and a drain obtained by applying a gate voltage will be too lowto be suitable for driving a liquid crystal device or the like. Inaddition, the above organic semiconductor layer has an On/Off ratio of100 or more.

FIG. 1 is an enlarged schematic view showing a part of the field effecttransistor of the present invention. The field effect transistor of thepresent invention is composed of a gate electrode 1, a gate insulatinglayer 2, a source electrode 3, a drain electrode 4 and an organicsemiconductor layer 5.

Materials for the gate electrode, source electrode and drain electrodeare not particularly limited, provided that they are electricallyconductive materials, and include platinum, gold, silver, nickel,chromium, copper, iron, tin, antimony, lead, tantalum, indium, aluminum,zinc, magnesium and alloys thereof; conductive metal oxides such asindium-tin oxide; and inorganic and organic semiconductors whoseconductivity is increased by doping or the like, for example, a siliconsingle crystal, a polysilicon, an amorphous silicon, germanium,graphite, polyacetylene, polyparaphenylene, polythiophene, polypyrrole,polyaniline, polythienylenevinylene and polyparaphenylenevinylene. Amethod of forming electrodes includes a sputtering method, a vapordeposition method, a printing method from a solution or a paste or anink jet method. Further, preferable electrode materials among the abovelist are those having a low electrical resistance at the contact surfacewith a semiconductor layer.

Any gate insulating layer may be used, provided that a solution of theporphyrin compound represented by general formula (1) can be uniformlycoated thereon, but those having a high dielectric constant and lowconductivity are preferred. Examples include inorganic oxides andnitrides such as silicon oxide, silicon nitride, aluminum oxide,titanium oxide and tantalum oxide; and organic polymers such aspolyacrylates, polymethacrylates, polyethylene terephthalate, polyimidesand polyethers. In addition, among the above insulating materials, thosehaving a high surface-smoothness are preferred.

In order to improve the uniformity on the insulating layer of thecoating film of a solution of the porphyrin compound soluble in organicsolvents represented by general formula (1), and to make the orientationof the film of the naphthoporphyrin compound represented by generalformula (2) uniform by heating, it is also possible to modify only thesurface of the insulating film. The method includes a dry treatmentusing ozone, plasma and a hexamethyldisilane gas and a wet treatmentusing a solution prepared by dissolving tetraalkoxysilane,trichlorosilane, a surfactant and the like in an organic solvent.

The structure of the field effect transistor according to the presentinvention is not limited to a thin film type but may be a cubic type.

EXAMPLES

Hereinafter, synthesis examples and examples will be shown. However, thepresent invention is not limited to these examples.

Synthesis Example 1 Synthesis of Bicycloporphyrin

Step (1)

A THF solution of 1 g (7.3 mmol) of anthranilic acid was dropped into amixed solution under reflux of 0.77 ml (8.0 mmol) of 1,3-cyclohexadiene,1.1 ml of isoamyl nitrite and 50 ml of tetrahydrofuran. Then, theresultant solution was heated and refluxed for 2 hours to remove THF,extracted with chloroform, washed with distilled water and saturatedsaline, dried with anhydrous sodium sulfate and purified with silica gelcolumn chromatography (chloroform) to obtain1,4-dihydro-1,4-ethanonaphthalene (0.89 g, 5.7 mmol, yield: 78%).

Step (2)

Under an argon atmosphere, a mixed solution of 0.156 g (1 mmol) of theobtained 1,4-dihydro-1,4-ethanonaphthalene, 30 ml of chloroform and 0.12ml (1.0 mmol) of sulfenyl chloride was cooled to −78° C. and stirred forone hour. Then, after the solvent was removed, the product was purifiedwith silica gel column chromatography (chloroform) to obtain2-chloro-1,2,3,4-tetrahydro-3-phenylthio-1,4-ethanonaphthalene (0.286 g,0.96 mmol, yield: 96%).

Step (3)

A solution of 0.286 g (0.96 mmol) of the obtained2-chloro-2,3,4-tetrahydro-3-phenylthio-1,4-ethanonaphthalene and 30 mlof chloroform was cooled to 0° C., added with 0.51 g of 75%m-chloroperbenzoic acid and stirred for two hours at room temperature.Then, the reaction solution was added with an aqueous saturated sodiumbicarbonate solution and extracted with chloroform. The extract waswashed with distilled water and saturated saline, dried with anhydroussodium sulfate and purified with silica gel column chromatography(chloroform) to obtain2-chloro-12,3,4-tetrahydro-3-phenylsulfonyl-1,4-ethanonaphthalene (0.3g, 0.91 mmol, yield: 95%).

Step (4)

A solution of 0.3 g (0.91 mmol) of the obtained2-chloro-1,2,3,4-tetrahydro-3-phenylsulfonyl-1,4-ethanonaphthalene and15 ml of anhydrous THF was cooled to 0° C., added with 2.5 ml of1,8-diazabicyclo[5.4.0]-7-undecene and stirred for 30 minutes. Thesolution is then added with a dilute hydrochloric acid solution,extracted with chloroform, washed with distilled water, an aqueoussaturated sodium bicarbonate solution and saturated saline and driedwith anhydrous sodium sulfate. The product was purified with silica gelcolumn chromatography (chloroform) to obtain1,4-dihydro-2-phenylsulfonyl-1,4-ethanonaphthalene (0.264 g, 0.89° mmol,yield: 98%).

Step (5)

Under an argon atmosphere, 0.296 g (1.0 mmol) of the obtained1,4-dihydro-2-phenylsulfonyl-1,4-ethanonaphthalene, 0.13 ml (1.15 mmol)of ethyl isocyano-acetate and 30 ml of anhydrous THF were charged andcooled to 0° C. Into the mixture, 1.7 ml of tert-BuOK (1 M THF solution)was dropped over two hours and the resultant mixture was stirred forthree hours at room temperature. After the completion of reaction, thereaction mixture was added with dilute hydrochloric acid, washed in turnwith an aqueous saturated sodium bicarbonate solution, distilled waterand saturated saline and dried with anhydrous sodium sulfate. Theproduct was purified with silica gel column chromatography (chloroform)to obtain ethyl-4,9-dihydro-4,9-ethano-2H-benz[f]isoindole-1-carboxylate(0.243 g, yield: 91%).

Step (6)

Under an argon atmosphere, a mixed solution of 0.243 g (0.91 mmol) ofthe obtainedethyl-4,9-dihydro-4,9-ethano-2H-benz[f]isoindole-1-carboxylate and 15 mlof anhydrous THF was cooled to 0° C., added with 0.10 g (2.7 mmol) of alithium aluminum hydride powder and stirred for two hours. Then, thereaction solution from which THF was removed was extracted withchloroform, washed in turn with an aqueous saturated sodium bicarbonatesolution, distilled water and saturated saline and dried with anhydroussodium sulfate. The reaction solution was filtered, purged with argon,light-shielded, added with 10 mg of p-toluene sulfonic acid and stirredfor 16 hours at room temperature. Further, the reaction solution wasadded with 0.11 g of p-chloranil and stirred for 16 hours at roomtemperature. The solution was washed in turn with an aqueous saturatedsodium bicarbonate solution, distilled water and saturated saline anddried with anhydrous sodium sulfate. The solution was concentrated andthen subjected to alumina column chromatography (chloroform) andrecrystallization (chloroform/methanol) to obtain bicycloporphyrinrepresented by general formula (3) (0.037 g, 0.045 mmol, yield: 20%).

Synthesis Example 2 Synthesis of a Bicycloporphyrin-Copper Complex

A solution of 0.1 g (0.12 mmol) of the above bicycloporphyrin and 0.065g (0.36 mmol) of copper acetate dihydrate in 30 ml of chloroform plus 3ml of methanol was stirred for three hours at room temperature. Thereaction solution was washed with distilled water and saturated salineand then dried with anhydrous sodium sulfate. The solution wasconcentrated and then recrystallized from chloroform/methanol to obtaina bicycloporphyrin-copper complex represented by general formula (4)(0.11 g, 0.12 mmol, yield: 100%).

Example 1

FIG. 1 shows the structure of the field effect transistor according tothe present invention.

First, an N-type silicon substrate was provided as the gate electrode 1.A silicon oxide film with a thickness of 5,000 angstroms obtained bythermally oxidizing the surface layer of the silicon substrate was usedas the gate insulating layer 2. Chromium and gold were vapor-depositedin turn thereon, and the source electrode 3 and drain electrode 4 wereformed by a conventional photolithography technique. Subsequently, thesubstrate was surface-treated by ozone treatment and then a 1% by weightsolution of bicycloporphyrin synthesized in Synthesis Example 1 (formula(3)) in chloroform was spin-cast on the substrate. Further, thesubstrate was heated at 290° C. to form the organic semiconductor layer5 composed of the compound of formula (5) shown below. The organic layerhad a film thickness of 60 nm, and the observation with an opticalmicroscope revealed that the maximum diameter of crystal grains was 2.5μm.

The field effect transistor with a channel length of 50 μm and a channelwidth of 10 mm was prepared according to the above procedure. TheV_(d)−I_(d) and V_(g)−I_(d) curves of the prepared transistor weremeasured using Parameter Analyzer 4156C (trade name) made by AgilentCompany (FIG. 2). Mobility μ (cm²/Vs) was calculated according toformula (1) below:I _(d)=μ(CiW/2L)(V _(g) −V _(th))²  (formula 1)wherein Ci denotes capacitance (F/cm²) per unit area of the gateinsulating film; W and L denote a channel width (mm) and a channellength (μm) respectively shown in the example; and I_(d), V_(g) andV_(th) denote a drain current (A), a gate-voltage (V) and a thresholdvoltage (V) respectively. Further, the ratio of Id for V_(g)=−80 V andId for 0V at V_(d)=−80 V was defined as an ON/OFF ratio. The obtainedresults showed that the transistor had a field-effect mobility of1.4×10⁻³ cm²/V·s and an On/Off ratio of 600.

The organic semiconductor layer 5 composed of a naphthoporphyrincompound was formed on a quartz substrate according to a similarfilm-forming condition. The observation of the ultraviolet-visible lightabsorption spectrum of the film using a spectrophotometer U3310 (tradename) made by Hitachi, Ltd. showed the absorption around 700 nm.

Example 2

A field effect transistor was prepared in which an organic semiconductorlayer composed of the compound of the formula (6) shown below was formedsimilarly as in Example 1 except that bicycloporphyrin described inExample 1 was replaced by a bicycloporphyrin-copper complex (formula(4)) synthesized in Synthesis Example 2. The field effect transistor hada field-effect mobility of 1.3×10⁻³ cm²/V·s and an On/Off ratio of 500.The organic semiconductor layer had a film thickness of 60 nm, and themaximum diameter of crystal grains was 2.0 μM.

The organic semiconductor film composed of the compound of the formula(6) prepared on a quartz plate showed the absorption around 700 nm.

Comparative Example 1

A field effect transistor was prepared similarly as in Example 1 exceptthat a vapor-deposited film of metal-free phthalocyanine commerciallyavailable (made by Sigma Aldrich Japan K.K.) was used for the organicsemiconductor layer in place of bicycloporphyrin described in Example 1.The vapor-deposited film was prepared at a substrate temperature of 150°C. and a vapor deposition temperature of about 380° C., and the filmthickness and vapor deposition rate calculated from the quartz-crystaloscillator was 100 nm and from 0.5 to 1.5 angstrom/s respectively. Thefield effect transistor had a field-effect mobility of 3.5×10⁻⁴ cm²/V·sand an On/Off ratio of 85. The maximum diameter of crystal grains was0.6 μm or less.

The present invention is not limited to the above embodiments, andvarious changes and modifications can be made within the spirit andscope of the present invention. Therefore to apprise the public of thescope of the present invention, the following claims are made.

1. A method of producing a field effect transistor comprising an organicsemiconductor layer, comprising a step of heating a coating filmcomprising a porphyrin compound represented by general formula (1):

wherein R₁ and R₂ each independently denote at least one selected fromthe group consisting of hydrogen, halogen, hydroxyl, and alkyl,oxyalkyl, thioalkyl and alkyl ester, each alkyl having 1 to 12 carbonatoms; R₃ denotes at least one selected from the group consisting of ahydrogen atom and an aryl group; and M denotes two hydrogen atoms, ametal atom or a metal oxide; to form as the organic semiconductor layera crystallized film of a porphyrin compound represented by generalformula (2):

wherein R₂, R₃ and M each denote the same as defined above.
 2. Themethod of producing a field effect transistor according to claim 1,wherein the coating film comprising the porphyrin compound representedby the general formula (1) is heated at a temperature range from 200 to350° C. to produce the compound of the general formula (2) therefrom. 3.A field effect transistor comprising an organic semiconductor layercomposed of a crystallized film of a naphthoporphyrin compoundrepresented by general formula (2):

wherein R₁ and R₂ each independently denote at least one selected fromthe group consisting of hydrogen, halogen, hydroxyl, and alkyl,oxyalkyl, thioalkyl and alkyl ester, each alkyl having 1 to 12 carbonatoms; R₃ denotes at least one selected from the group consisting of ahydrogen atom and an aryl group; and M denotes two hydrogen atoms, ametal atom or a metal oxide, wherein the crystallized film has crystalgrains having a maximum diameter of 1 μm or more.
 4. The field effecttransistor according to claim 3, wherein the organic semiconductor layercomprised of the naphthoporphyrin compound represented by the generalformula (2) has a strong absorption at 650 nm or longer.
 5. The fieldeffect transistor according to claim 3 or 4, wherein in thenaphthoporphyrin compound represented by the general formula (2), R₂ isa hydrogen atom.
 6. The field effect transistor according to claim 3,wherein in the naphthoporphyrin compound represented by general formula(2), R₃ is a hydrogen atom.
 7. The field effect transistor according toclaim 3, wherein in the naphthoporphyrin compound represented by generalformula (2), M represents two hydrogen atoms.
 8. The field effecttransistor according to claim 3, wherein in the naphthoporphyrincompound represented by general formula (2), M represents one copperatom.
 9. The field effect transistor according to claim 3, wherein theorganic semiconductor layer has a field effect mobility of 1×10⁻³cm²/V·s or more and an On/Off ratio of 100 or more.
 10. A field effecttransistor comprising an organic semiconductor layer composed of acrystallized layer of a naphthoporphyrin compound represented by generalformula (2):

wherein R₁ and R₂ each independently denote at least one selected fromthe group consisting of hydrogen, halogen, hydroxyl and alkyl, oxyalkyl,thioalkyl and alkyl ester, each alkyl those having 1 to 12 carbon atoms;R₃ denotes at least one selected from the group consisting of a hydrogenatom and an aryl group; and M denotes two hydrogen atoms, a metal atomor a metal oxide, wherein the crystallized film has a strong absorptionat 650 nm or longer.